1. Field of the Invention
The present invention relates to a network control system for both an asynchronous network type control system and a synchronous network type control system. In particular, the invention relates to a control system in which in a case where a controlled object process and a control apparatus are connected to each other via an asynchronous network, the control apparatus performs a control by calculating control variables for the controlled object process on a real-time basis.
2. Description of the Related Art
In conventional control systems, it is necessary to design means for calculating an optimum value of a control variable in accordance with dynamic characteristics of a controlled object process and implement it in a control apparatus in advance, and to execute a series of processes such as observation of a process variable, control calculations, and output of a value of a manipulated variable (in this specification, "manipulated variable" is as same meaning as "control variables") on a real-time basis, that is, at constant intervals. Therefore, it is necessary to construct a special network system (synchronous network) in which real-time operation is taken into account.
On the other hand, to keep the communication line cost low, extra costs are needed for development and maintenance for that purpose. In particular, where a controlled object process and a manipulation station including a control apparatus need to be installed at respective places that are very distant from each other, it is necessary to use a dedicated line in which real-time operation is taken into consideration. This requires a high communication line cost.
If an asynchronous network that is low in communication line cost is used, because it is a public line, there occurs a transmission delay that varies depending on a network state; for example, a large transmission delay occurs during a congested time. As a result, a variable, useless time occurs in a one-circulation path of a process signal or a control signal starting from a controlled object, reaching a control apparatus, and returning to the controlled object. This causes a problem that the control performance and the stability are lowered.